Manhole covers made of cast iron make up the bulk of access covers in urban streets around the world. These covers have a proven record of durability, strength, and functionality. The high density of iron results in a cover of substantial weight preventing cover displacement during flooding conditions and minimizing the occurrence of theft.
Recently, gas or steam explosions below a street can cause manhole covers to become dangerous projectiles. For example, a cast iron manhole cover can weigh between 85 and 300 pounds, and explosions can propel the cover from 1 foot to 50 feet into the air. Often during various events particularly in large cities, utilities will temporary weld manhole covers to their frames to prevent access in and out of the manholes.